Process for preparing photographic gelatin



sept B966 G. H. KLINGER PROCESS FOR PREPARING PHOTOGRAPHIC GELATIN Filed April 26, 1963 w w n INVENTOR. @MA/m56 A4 Kaff/55 BY j@ u 3,271,174 PROCESS FUR PREPARING PHU'IOGRAPHIC GELA'IIN Guenther H. Klinger, Binghamton, NX., assigner to General Aniline & Film Corporation, New York, NY., a corporation of Delaware Filed Apr. 26, 1963, Ser. No. 276,04) 4 Claims. (Cl. l06-12S) The present invention relates t a process for preparing inert gelatins and, more particularly, to the preparation of such gelatins of uniform and predeterminable sensitivity for use in various types of photographic emulsions.

There has long been a need for an inert gelatin in the photographic industry. Inert gelatins have been produced, but their production has involved diculties. Emulsion ymakers for some time have sought for a simple effective method to remove the sensitizing compounds from photographic gelatins which would not destroy the valuable properties of the inertized gelatin. Many proposals have been made to remove the sensitive materials and then add sensitizing materials such as alkali metal thiosulfates in known amounts. Such a procedure, when properly effected, has the advantage that gelatins originally too high in sensitizing compounds and which are ordinarily unfit for photographic purposes can be used. An object of this invention is to render the gelatin essentially inert by a controllable method without harming its desirable physical and chemical properties.

Manifestly, it would be advantageous to be able readily to make photographic emulsions of uniform quality, sensitivity and speed from gelatins obtained from different raw materials. Gelatin is an organic material which is obtained from natural sources and frequently from a variety of sources. For example, some of the gelatins used in photography are obtained from calf hide and ears, pigskin, decalcified gelatin or the like. These imaterials differ considerably in their content of active sulfur sensitizers. Hence, there is often a considerable variation in the properties of the gelatin used for photographic emulsions. Some types of gelatin may have a considerable proportion of photographically active sensitizing ingredients and others may have much less. Also, the type of sensitizer present may vary considerably; These sensitizers vary in composition and while some of their compositions are not fully understood, they usually are sulfur compounds, although frequently of types and concentrations not accurately determinable.

Because of the variations in quality of gelatin, it has heretofore been difficult to prepare photographic emulsions of uniform properties by following a set formula for their preparation. This invention makes it possible to substantially inactivate the photographically active materials present in the gelatin regardless of its source or origin. This procedure may also be applied not only to gelatins, but also, at least to some extent, to other emulsion-forming materials such as casein, zein, albumin and Various other proteins.

Past efforts to desensitize gelatin or to make it photographically inert have been physical as well as chemical. With a truly inert material to start with, it is obvious that desensitizirng materials of known type, quality and quantity may be added and the above general objectives thus obtained. In general, however, t-hese prior art methods have not been entirely successful. For example, such physical methods as prolonged Washing of a swelled gelatin with water extracts preferentially the lower molecular constituents of the gelatin and, therefore, changes its physical properties. On the other hand, treatment with adsorbents not only removes sensitizers, but restrainers as well and alters the characteristics of the gelatin coml'ilhill Patented Sept. 6, i966 pletely. In addition, the latter method is quite uneconomical.

The chemical methods which have been resorted to usually consist of treating the gelatin with an oxidizing agent, such as hydrogen peroxide or a peroxy acid. The oxidizing agent must be present in excess to assure compiete destruction of the sensitizer. The excess is removed after completion of the treatment with a reducing agent, such as a sulite.

This chemical treatment of gelatin modifies the chemical and physical properties of the material to a great extent. For example, changes may occur in the isoelectric point, setting point and viscosity. Such changes indicate that the protein molecule has experienced a partial degradation. Hand in hand with such degradation goes a decrease of the 4physical properties of the coated layers in which the gelatin is employed.

An additional diculty lwith the chemical process is that the excess of the oxidizing agent must be carefully removed. In doing so usually a small excess of sulfte ion is employed to ensure complete removal of the oxidizing agent. Gelatin which has been inertized in this fashion shows on the average an unusually high amount of sultite which is photographically active. The end, therefore, hardly justifies the means.

So far as the prior art methods `are concerned, they are injurious in many instances and as a rule, they are not entirely effective in removing the sensitizing materials in any case. Hence, an object of this invention is to provide an effective chemical method for treating gelatin and other protein materials whereby the photographically active ingredients are rendered inactive without material change in other properties of the gelatin. A further object is to provide a very simple method for the manufacture of inert photographic gelatin with a minimum of manipulation.

The new method consists of treating the gelatin with active unsaturated chemical compounds. These are of a type which add to mercapto groups which are usually present in the sulfur compounds that are effective sensitizing agents in gelatins, sold on the open market. In so reacting with these active unsaturates, there are formed aryl or alkyl suldes which `are photographically inert. These materials, although they may remain in the gelatin, are not harmful.

Unsaturated organic compounds contemplated for use in the present invention and which can undergo an additional reaction with mercaptans, are as follows:

(l) N-Arylor N-A'lkylmaleimides where Aryl is a phenyl, toluyl or naphthyl radical and Alkyl is methyl, ethyl, propyl, butyl, decyl and the like;

(2) Vinyl esters such as o H2C=CC% OR when R is Alkyl, i.e., methyl, ethyl, propyl or butyl, or Alyl, i.e., phenyl, Itoluyl or naphthyl;

(3) Vinyl ethers such as CHzCH-O-R wherein R is Alkyl such as methyl, ethyl, propyl, butyl or the like, or Aryl such as phenyl, toluyl or naphthyl;

(4) Dihydropyrane of the formula and its derivatives such as 3,4-dihydro-2-methoxy-4- methyl pyrane, 3,4-d-ihydro-Z-methoxy-l-phenyl pyrane, 3,4-dihydro-Z-methoxy-t-benzimidazole pyrane, 3,4-dihydro-2-isoproxy-4-butyl pyrane, 3,4-dihydro-2-butoxy- 4-ethyl pyrane, 3,4-dihydro-2-butoxy-4-benzimidazole pyrane;

3 (5) Dihydrofurane of the formula (9) Ortho-iodosobenzoic acid 4 )vooon Another compound similar to the first two listed abo-ve is N2,4d initroanalinomaleimide. This material, and a method for its preparation, are described in 1961 JACS at page 2810. Various other maleimide derivatives of this type also may be used.

The dihydropyranes are prepared according to the method described in Organic Synthesis, vol. 34, Page 29 (1954) and the dihydrofuranes by the method descni-bed in Angewandte Chemie No. 22, page 865 (1960). The yields of the latter are generally higher than 70%.

The preferred compounds are the monomaleimides, particularly those listed in the first group above.

The method of this invention for manufacture of inert gelatins has the follow-ing advantages:

a. The inertization procedure consists simply in adding the dissolved chemical to the gel solution.

b. The physical properties of the gelatins are essentially unchanged or changed only to a minor extent.

c. An excess of the reactive compound can be used and left in the gelatin without harm. Addition of a larger amount of sulfur sensitizer later, such as a thiosulfate, may ,be made during the chemical ripening to take care of any excess of the inertizer.

d. The restrainers present in the gelatin are not affected by the inertizing compound.

e. The inertizat-ion treatment can be included in the normal gelatin manufacturing process. For example, it maybe included in the extraction process and, therefore, requires no separate processing step.

FIGURE 1 uses an untreated gelatin in an amount of 100% in a medical X-ray emulsion. The two curves involved disclose speed variations in 3 minutes development and fog variations in 12 minutes development. The fog density was multiplied by 100 in order to portray the density on the graph.

FIGURE 2 uses a bone gelatin in an amount of 100% in .a medical X-ray emulsion. The curve depicts speed variation in 3 minutes development and fog variation in 12 minutes development. In FIGURE 2 curve B was obtained while utilizing artificial sensitizing in the afterripening process. For this purpose there were employed 1 to 6 milligrams of a 110% solution of sodium thiosulfate and 2 cc. of 5% sodium sulfite per kilo of emulsion. The great increase in speed is clearly indicated in FIGURE 2. As in FIGURE l, the fog density was multiplied by 100 in order to depict such density on the graph.

Example 1 A 4.5% solution of Nphenylmaleimide in methanol solution was added to the reconstitution gel of a bromoodide emulsion. The emulsion was ripened at a pBr 4 of 3.5 without addition of sensitizing compounds such as those of gold or sulfur. Table No. 1 lshows that small amounts of the maleimide (about 0.7 milligram or 1 milliliter of solution per gram of gelatin) apparently does not interfere with ripening. On the contrary, it appears to have a rather -beneficial influence on the clarity of the emulsion. Largeramounts (such as 7 milligrams per gram of `gel or 10 milliliters of solution) cause desensitization.

TABLE L ADDITION OF 11 4.5% SOLUTION OF N-PHENYL- MALEIMIDE TO AN EXPERIMENTAL EMULSION AT RECONSTITUTION Speed 15.9 10. 2 1. 3 15 Fog 0. 09 0. 08 0. 0G 0.21 Boiled Type Ml. Maleinide per 61 1 5 10 0 Emulsion.

g. gelatin. Speed 5. 5 8. 7 3. 8 4. 9 Fog 0. 32 0.08 0. 13 0.57 Ammonia Type Ml. nflaleimide per 61 1 5 10 0 Emulsion.

g. gelatin.

Table II shows the effect of a 4.5% N-phenylmaleimide solution when added to a coating final. It is evident from this table that large amounts of the reagent, such as 1l to 22 mg. per gram of gelatin, which caused desensitization when added to an emulsion after completion of the ripening, do not desensitize the emulsion layer when added as a coating final.

TABLE II.-EFFECT OF USING N-IHENYLMALEIMIDE IN 4.5% METIIANOL SOLUTION WHEN ADDED AS A COATING FINAL TO 500 g. OF GELATIN AT END OF RIPENING STEP Sample N0. 2,430 2,431 2,432 2,433 2,434

Amount added, mL... 0 2 4 10 2O Example 2 An inert gelatin was prepared using N-phenylmalemide as follows: 600 grams of gelatin (Koeptf 3773) were dissolved in 1800 ml. of Water at 40 C. A solution of 2.1 grams of N-phenylmaleimide in 200 cc. of methyl alcohol was added. The gelatin solution was poured into stainless steel trays and dried at 37 C. An X-ray emulsion was prepared from this gelatin. It showed no light sensitivity when ripened in the absence of sulfur sensitizers. Addition of conventional sulfur sensitizers brought the speed back to within one stop of the emulsion prepared with the untreated gelatin.

Example 3 TABLE III Treated (Koepti) 6.5 1n g. N-Phenylmale- (Koeptl) Untreated imide per gram 0f Gel Example 4 Additional experiments on N-phenylmaleimide inertized gelatins showed that this inertization procedure may have the beneficial effect of moderately increasing the setting point of the treated gelatins. The setting points of the N-phenylmaleimide inertized gelatins increased from 1.2 to 11.7%, depending on the particular gelatin. Conversely, inertization with peroxyacetic acid usually lowers the setting point.

The following table shows the setting points of gelatins before and after N-phenylmaleimide treatment. All vthese gelatins were treated with 3.7 mg. N-phenylmaleimide per gram of gelatin.

It is known that some of the maleimides, when added in substantial quantities to gelatin, may serve as cross linking agents and may be effective to harden the gelatin considerably. In this connection reference is made to IACS, vol. 81, pages 1187-90, published March 5, 1959. This article describes the use of dimaleimides for cross linking of various types of polymers and suggests that one can convert gelatin to an insoluble gel by using this material in an aqueous solution. It suggests also that the cross linking presumably involves free amino groups present in the lycine, arginine or histidine units of the gelatin and gives examples of the insolubilization of gelatin by various processes. However, in the quantities used for purposes of desensitizing and particularly when used under the conditions described above, the physical properties of the gelatin are not substantially changed by the monomaleimides enumerated above.

The present invention is distinguished from the type of operation which involves hardening of the gelatin. The purpose of the monomaleimides, according to this in- Vention, is merely to desensitize the gelatin by removing the relatively small proportions of sulfur sensi-tizers. This is preferably accomplished before the silver halide and other ingredients are added to the emulsion.

It will be understood that, while the maleimides enumerated above are a preferred class of compounds, any of the compounds mentioned above have utility so long as they contain activated double (or unsaturated) bonds which are capable of tying up the sulfur by addition, thus rendering it inert by converting it -to the insensitive sulfide. In other words the examples may be reproduced while using any of the inertizing compounds mentioned in the specification.

Various modifications may be made in the process, as will readily occur to those skilled in the art. Therefore, it is intended by the claims which follow to cover the obvious modifications and variations which would commend themselves immediately to those `skilled in the art, as far as the state of the prior art permits.

What is claimed is:

1. The process of preparing inert photographic gelatin which contains an active sulfur sensitizer, which cornprises desensitizing the gelatin without a substantial change in its physical properties by Iltreating the gelatin containing said sensitizer with 0.01 to 1% by weight, based on the gelatin, of an unsaturated thiolophilic compound capable of forming an addition compound with a mercapto compound and having at least one activated double bond and selected from the group consisting of N-aryl maleimides, N-alkyl maleimides, vinyl ethers, vinyl esters, dihydropyranes, dihydrofuranes, ethylvinylsulfone, divinyl sulfone, 2,5-diethyleneimine-quinone and ortho iodosobenzoic acid to render said sulfur inert.

2. Process according to claim 1 wherein the thiolophilic compound is -a mono-maleimide.

3. Process according to claim 1l wherein the thiolophilic compound is N-aryl maleimide.

4. Process according to claim 1 wherein there is subsequently added to the gelatin a known active sulfur sensitizer to adjust the sensitivity to the desired value.

References Cited by the Examiner UNITED STATES PATENTS 2,367,511 1/1945 Lowe et al. 96-111 2,392,294 1/1946 Rust et al 260-609 2,423,773 7/1947 Hart et al. 260-118 2,449,980 9/1948 Damschroder et al. 260--118 2,994,611 8/1961 Heyna et al. 96-111 3,061,436 10/1962 Himmelmann et al. 96-111 3,186,846 6/1965 Ryan 96-94 NORMAN G. TORCHIN, Primary Examiner.

A. E. TANENHOLTZ, I. RAUBITSCHEK,

Assistant Examiners. 

1. THE PROCESS OF PREPARING INERT PHOTOGRAPHIC GELATIN WHICH CONTAINS AN ACTIVE SULFUR SENSITIZER, WHICH COMPRISES DESENSITIZING THE GELATIN WITHOUT A SUBSTANTIAL CHANGE IN ITS PHYSICAL PROPERTIES BY TREATING THE GELATIN CONTAINING SAID SENSITIZER WITH 0.01 TO 1% BY WEIGHT, BASED ON THE GELATIN, OF AN UNSATURATED THIOLOPHILIC COMPOUND CAPABLE OF FORMING AN ADDITION COMPOUND WITH A MERCAPTO COMPOUND AND HAVING AT LEAST ONE ACTIVATED DOUBLE BOND AND SELECTED FROM THE GROUP CONSISTING OF N-ARYL MALEIMIDES, N-ALKYL MALEIMIDES, VINYL ETHERS, VINYL ESTERS, DIHYDROPYRANES, DIHYDROFURANES, ETHYLVINYLSULFONE, DIVINYL SULFONTE, 2,5-DIETHYLENEIMINE-QUINONE AND ORTHO IODOSOBENZOIC ACID TO RENDER SAID SULFUR INERT. 